The present invention generally relates to cellular, mobile, radiotelephone systems using a type of coded transmission known as CDMA. More particularly, the present invention relates to methods and systems for reducing adjacent channel interference in CDMA radiotelephone systems.
Cellular mobile phone systems comprise a number of fixed or base stations that are in communication with a number of portable or mobile stations. The information flow from the base-to-mobile is termed the downlink, while the information flow from the mobile-to-base is termed the uplink.
Current cellular mobile telephone systems primarily employ Frequency Division Multiple Access (FDMA) and analog frequency modulation for speech transmission. Systems will soon be installed worldwide using Time Division Multiple Access (TDMA) and digital transmission of speech to provide higher capacity. Code Division Multiple Access (CDMA) is a well-known technique that has principally been used for military satellite communications because of its jamming resistance. CDMA permits several signals to simultaneously use the same frequency even in the same cell, in contrast to FDMA systems that require different signals in the same cell to use different frequencies, and TDMA systems that permit use of the same frequency but not at the same time.
CDMA systems have not previously been favored for landmobile communications because of the so-called near/far ratio problem. In landmobile communications, the ratio of the distance of one mobile station to a base station to the distance of another mobile station to the base station can be relatively high. This can lead to a large difference in the signal strength that the base station receives from both mobile stations because signal propagation loss varies roughly as a function of the fourth power of the distance between a mobile station and a base station. Since mobile stations in CDMA systems can transmit on the same frequency at the same time, signals received at the base station having relatively high signal strengths tend to interfere with those having lower signal strengths. This is not a problem in satellite communications since a geostationary satellite is equidistant from all points on the earth.
Recent advances in CDMA techniques, such as subtractive demodulation, have resulted in more attention now being given to CDMA techniques for landmobile radio applications. In subtractive CDMA demodulation, signals received at the base station are demodulated in signal-strength order from highest to lowest, and decoded signals are subtracted out of the composite signal before attempting to decode the next weakest signal. Subtractive demodulation combined with power tapering, which is described below, on the downlink and appropriate selection of mobile power on the uplink, controls the near/far ratio problem with respect to signals on the same frequency. These techniques, however, do not compensate for potential interference from signals on other frequencies which would be too complicated to decode and subtract out. Therefore conventional CDMA systems, as well as subtractive demodulation systems, do not provide any particular benefits in terms of adjacent channel interference tolerance.
Greatest capacity in CDMA systems can be obtained when the power used to transmit to a mobile station on the downlink is tailored according to the distance from the mobile station to the center of the cell since this will reduce interference. Higher power is transmitted to mobile stations further away while lower power is transmitted to those mobile stations near the cell center. The consequence of this technique, called power tapering, is that the weaker signals will be more sensitive to interference from energy in the adjacent channels than will be the stronger signals. It is unfortunately the weaker signals' greater sensitivity that determines the quality of channel filters used in receivers to suppress adjacent channel energy and, therefore, the benefits of power tapering can be offset by the expense of the channel filters.
Another factor in determining the required adjacent channel suppression is the fading margin that must be allowed. Even if adjacent channel signals are transmitted from the same base station antenna as the desired signal, these adjacent channel signals will fade in a manner uncorrelated to the fading of the desired signal, so that the adjacent channel signal can at times become stronger while the desired signal fades weaker. If 0.1 percent fading tolerance must be allowed for, a fading margin of over 30 dB must be added to the required adjacent channel suppression requirements.